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Programming in C– An Overview

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C-Programming General Purpose Language Developed by Dennis Ritchie of Bell Labs

(1972). First used as The Systems Language for The UNIX operating system.

Why ‘C’1. C is a SMALL Language

Small is beautiful in programming Fewer key words but powerful

2. C is the Standard Developmental Language for personal computers.Much of : MS-DOS & OS/2

Many windowing packages, Data Base PGMS, Graphic

Libraries & other Large Application Packages are written in ‘C’

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3. ‘C’ is Portable - easily moved from machine to machine.

Provides a standard library of functions that work the same on all machines.

Built in Preprocessor - helps isolate any system dependent code.

4. ‘C’ is Terse - very powerful set of operators.

Can accomplish in 1 statement what might require many statements in another language.

A Terse language explicitly magnifies the underlying productivity of the programmers.

5. ‘C’ is Modular - Supports Structured Programming.

Functions - Internal - External

Supports user - Defined Libraries of Functions

Supports Privacy by using “Static” storage Class within files.

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Lexical Elements

• C is a language.– Characters + Punctuators + Rules.

• C Program is viewed as a sequence of Tokens separated by White Space (i.e., spaces, tabs, new lines).

– Keywords.

– Identifiers.

– Constants.

– String constants.

– Operators.

– Punctuators.

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Keywords

• Reserved words.

• Cannot be redefined or used in other contexts.

• Case sensitive.

• See pg. 46 in your text.

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Identifiers

• The names of variables, functions, labels and other user-defined items are called identifiers.

• May be any length.– ANSI C – First 31 characters are recognized.

• Cannot be the same as a keyword or library function.

• Case sensitive.– E.g., count, Count and COUNT are three separate

identifiers.

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• The first character must be a letter or an underscore, and subsequent characters must be either letters, digits or underscores.

Correct Incorrect

Count1 1count

test23 hi!there

high_balance high . . . Balance

• Identifiers should be chosen to reflect their use in the program.

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Constants

• Integer A) Decimal 0-9

• B) Octal 0-7

• C) Hexidecimal 0-F

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Floating Point: Need . Or E format

23.65 .897 2e+03 2.34e-05

Character: Any single printable character in single

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String Constants

• Character(s) enclosed in Double Quotes

• “A” and ‘A’ or not the same.

• “A” is actually followed by the Null character.

“Hello” is really “Hello\0”

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“a string of text”

“” // the null string

“ ” // a string of a blank character

“ a = b + c; ” // nothing is executed

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Operators

• Arithmetic Addition +• Subtraction -• Multiplication *• Division / • { int/int – int}

• Modulus %

• 5%3 = 2 Integer Remainder• 3%9 = 3

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• Relational Operators:

< <= > >= != ==

Allow us to create Boolean expressions

which can control program execution.

Logical Operators: Not !

And &&

Or ||

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Sometimes the meaning of the operator must be its determined by context.

a) scanf (“% d”, variable);

b) x = y % z;

a) % a conversion character

b) % a modulus operator

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Punctuators

• They are located by the compiler as tokens and are used to separate language elements.

Parentheses ( ) Braces { }

Commas , Semicolons ;

int main(void){

int a, b=3, c=6;a = 17 * (b + c);…

}

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Terms

• Source code– Text of a program.– The input to the C compiler.

• Object code– Translation of the source code of a program into

machine code. Output from the Compiler.– The input to the linker.

• Linker– A program that links separately compiled modules

into one program.– The output is an executable program.

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• Library– The file containing the standard functions that

a program can use.

• Compile time– The time during which a program is being

compiled.

• Run time– The time during which a program is executing.

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Compilation

• A compiler translates source code into machine code.

• One high-level instruction can correspond to several machine instructions.

Ex: x = (y + 3) / z;

• Control structureswhile, repeat, if-then-else, case, …

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• Procedures/subroutines/functions.

Must create machine language code to:– Copy input parameter values .– Save return address.– Start executing at beginning of invoked

routine.– Copy output parameter values at end of

procedure.– Set PC to return address.

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• Lexical Analysis: Break up command syntax into logical components, called tokens, and discard comments, spaces, etc. Ex: total=sum + 55.32 contains five tokens,

total, =, sum, +, and 55.32

• Parsing: Decide how tokens are related.

Ex: sum + 55.32 is an arithmetic expression, total = sum + 55.32 is an assignment

statement.

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• Code generation:

Generating machine instructions to

implement each high-level command.

It is important to optimize the code.

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1. X = Y+Z;

2. W =X+Z;

After line 1, X and Z are already in registers, so no need to re- load the registers for line 2, therefore execution is faster. Less retrieval time.

The resulting machine language program,called object code, is written to disk.

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Library and Linking

• Most programs include calls to various functions contained in C's standard library. The object code(from C compiler) contains “holes” due to these missing parts.

• A Linker links the object code with the code for the missing functions to produce an executable image(load module).

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Library and Linking

Linker: combines the result of compiling different pieces of the program separately. If pieces refer to each other, these references cannot be resolved during independent compilation.

procedure p main

Refers to x

Declares x

Invokes p

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Two separate files

int count;                       #include <stdio.h>

void display(void);        extern int count; int main(void)    void display(void){ { count = 10; printf(“%d”, count); display(); } return 0;}

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C’s Memory Map

• Separate compilation.

• Conceptualized memory map of a C program. Memory for the program, memory for the data.

StackHeap-DynamicGlobal variablesProgram code

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A Hello World Program

/*The traditional first program in honor of Dennis Ritchie who invented C at Bell Labs in 1972.*/

#include <stdio.h>

int main(void) //main function{ printf(“Hello, world!\n”); return 0;}

Hello, world!

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Format of the Function:

Preprocessor Directives //Here we include any libraries required by the program.

:

int main ()

{

Declarations ; //Here we list any data variable declarations.

Body of Function //Here is the source program version of the algorithm.

}

Main Function: Program will have 1 main function where Execution Begins & Ends.

Pre-Defined Functions: Already Exist - Part of the Language. Programmer uses them to do predefined

Activities - Input/Output.

User-Defined Functions: Functions written by the programmer. Created to do specific task(s).

e.g. - Given Hours worked & payrate;

Calculate Employee salary.

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Preprocessing directives

• Before compilation takes place the preprocessor modifies the source code that is input to the compiler.

includes files - #include define constants - #define

• All preprocessor directives begin with a # sign.• Each preprocessing directive must be on its own

line. #include <stdio.h> #include <stdlib.h> //wrong

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#include

• General Syntax:

– #include <filename> e.g. #include <stdio.h>The preprocessor looks for the file only in standard places.

– #include “filename” e.g. #include “utility.h”The preprocessor first search the file in the current directory and then in other system-dependent places.

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• General Syntax: Symbolic Constant

#define macro-name char-sequence

e.g. #define PI 3.14

• The preprocessor changes all occurrence of the identifier PI to 3.14.

e.g. printf(“PI = %f\n”, PI) printf(“PI = %f\n”, 3.14)

• A symbolic constant is easily changed later.• By convention, a symbolic constant is written in

capital letters.

#define

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#define ONE    1#define TWO    ONE+ONE

#define E_MS "standard error on input\n"printf(E_MS);

#define XYZ this is a testprintf("XYZ"); xyz

standard error on input

#define LONG_STRING "this is a very long \string that is used as an example"

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# define: can be placed anywhere in your source code.

usually place at the beginning of your source code before main( ) function.

Where you place them determines their SCOPE.

SCOPE: The segment of code that “knows about” the existence of the symbolic constants and/or

macros you have created.

e.g., If place at the beginning of your source file, then the SCOPE is the entire

file.

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Header Files: Are joined with your source code by the Preprocessing Directive #.

Why - often reason is that they contain the FUNCTIONAL PROTOTYPES (FP’s) of functions that the program wishes to call. - If the FP’s are not available then the program can’t call (use) those functions.

Functional Prototype: The “Heading” of the function.

Contains the Name and return type of the function. The number & type of values that may be passed to the function (Formal parameters).

The Compiler needs the FP’s of invoked functions to perform its job.

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Comments• Multi-Line comments

– May be placed anywhere in a program.

– May not be nested.

/* this is a multilinecomment */

x = 10+ /* add the numbers */5; //correct

/* this is an outer comment   x = y/a;  /* this is an inner comment - and causes an error */*/ //wrong

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• Single-Line Comments

– A single-line comment can be nested within a multi-line comment.

– All but the most obvious functions should have a comment at the top that states what the function does, how it is called, and what it returns.

// this is a single-line comment

/* this is a // test of nested comments. */

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A Depth Program#include <stdio.h>

int main(void)

{

int inches, feet, fathoms; //declarations

fathoms = 7;

feet = 6 * fathoms;

inches = 12 * feet;

printf(“Wreck of the Hesperus:\n”);

printf(“Its depth at sea in different units:\n”);

printf(“ %d fathoms\n”, fathoms);

printf(“ %d feet\n”, feet);

printf(“ %d inches\n”, inches);

}

Wreck of the Hesperus:Its depth at sea in different units: 7 fathoms

42 feet 504 inches

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Declaration

• A variable is declared to be of a type. This enables the compiler to:

– Set aside the appropriate amount of memory to hold the variable.

– Defines what operations are permissible with that variable.

• All variables must be declared before they are assigned values and used in statements.

• Declaration end with a semicolon.

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Assignment• Allows the work and processing of data to achieved.

variable; Variable = constant;

function call;expression; [contains any/all of above

joined by operators]

• Multiple assignments. Right to Left Association:1. C = 0

A = B = C = 0; 2. B = (C = 0)3. A = (B = (C = 0))

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Compound Assignments

+=, -=, *=, /=, %=

General Form:Variable += Exp; Variable = Variable + Exp;Variable -= Exp Variable = Variable - ExpVariable *= Exp Variable = Variable * ExpVariable /= Exp Variable = Variable * ExpVariable %= Exp Variable = Variable % Exp

x += 10; x = x +10;x -= 100; x = x - 100;

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Data Types: See more in Chap. 6

Variables - Locations in memory we create which can hold data.

Variables have a specific type. Type defines what sort of data is allowed

to be stored in the variable.

Integer Types: int, short int, long int

Real Types: float, double, long double

Text Types: Char 1 keystoke (1 symbol) Stringmultiple characters

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Initialization• When variables are declared, they may also be

initialized. Typically, constants or constant expressions are used to initialize a variable. General Syntax

type variableName = constant;

A) Integer Constant:e.g. x = 100; correct 100 = x; wrong

B) Real: 1.63 double1.63F float1.63L long double

e.g. float x, y;

x = 1.0; y = 2.0; x/y = 0.5;

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C) Char Single symbol in single quotes.

e.g. Char Letter;

Letter = ‘A’.

Letter = ‘9’;

D) String Multiple symbols in double quotes.

e.g. “This is a string constant”

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• General Syntax:

– scanf (“format string”, Arguments)• Interactive Input: get data from user entering via keyboard.

– fscanf (f, “format string”, Arguments)• File Input: get data that is stored in file F.

•

– sscanf (s, “format string”, Arguments)• String Input: get data from string S.

• Return value: If error occurs, return value is EOF (usually -1), Otherwise returns the number of successful assignments of data to the arguments.

Input Predefined Functions

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• Unsuccessful Input:

– EOF (End of File) is reached before ALL arguments have been assigned data, if so the value EOF (-1) is returned.

– Data Mismatch: The input data does not match the control string specifications. (e.g., indicated data would be integer and the data actually entered was REAL).

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• SCANf - Interactive Input

– User enters data with keyboard when program interrupt occurs.

– Input data treated as an input stream of characters.

– Arguments of the scanf are addresses.

scanf(“%d”, &x);

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String Input#include <stdio.h>

int main(void){  char str[80];   printf("Enter a string: ");  scanf(''%s", str);

  printf("Here's your string: %s", str);  

return 0;}

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Output Predefined Functions• General Syntax:

– printf (“control string”, Arguments)• Writes to terminal.

– fprintf (f, “control string”, Arguments)• Writes to another file.

– sprintf (s, “control string”, Arguments)• Writes to string s.

• Return Value: If EOF occurs, return value is -1, Otherwise returns the number of successful assignments of data to the arguments.

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Format Specifiers

Contain Conversion Specification Characters:e.g., d - decimal data c - character

f - floating point s - string

printf("I like %c %s", 'C', "very much!");

Contain Text:

int Sum = 500;

printf (“The Sum is %d”, Sum);

New Line - printf (“hello, \n world”);

Tab - printf (“hello, \t world \n”)

The Sum is 500

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ASIDE: scanf float use %fdouble use %lf

printf float or use %fdouble

Format Specifiers (cont.)

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Field Width Specification

printf (“more numbers: %7.1f %7.2f %7.3f”, 4.0, 5.0, 6.0);

Field width. Precision

# of columns output # of decimal digits to right

including dec. pt. of decimal pt.

more numbers:_ _ _ _ 4.0 _ _ _ 5.00 _ _ 6.000

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Character Data:

printf (“%c%5c/n%4c”, ‘x’, ‘y’, ‘z’);

x_ _ _ _ y

_ _ _z

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Looping - Iteration

• Categories: Conditional Loops: while do-while

Counting Loop : for

• While Statement:

while (Expression) {

Body of Loop

}

while (scanf(“%d”, &d)==1){

printf(“%d”, d);}

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How Terminate Loop?• Dummy Data Logic:

– User enter a value that can’t be successfully converted.

• End-of-file Logic:– Indicate all data has been entered by typing

EOF signal.UNIX ctrl + d

MS DOS ctrl + z

• break - Exit from Loop immediately.

• continue - Jump to Bottom of Loop.